"It’s called Teaching With Calvin and Hobbes, and it was published in 1993 by a small press in Fargo, North Dakota. Apparently Watterson was persuaded to make an exception to his general refusal to license his strip after the authors communicated with him directly, and described how using the strip had assisted in their lessons for children with learning disabilities. There were only 2,500 copies printed, and these days, their rarity means they’re worth a small fortune — a copy sold for $10,000 in 2009."